Zeolite from the class of mazzite, process for its synthesis, and its use as a catalyst

ABSTRACT

This invention relates to a new zeolite from the class of mazzite, containing a fraction of non-exchangeable potassium cations, and also to a process for its synthesis. The crystallinity and the adsorption capacity of the protonated form of this zeolite are maintained after a prolonged hydrothermal treatment.

This is a division, of application Ser. No. 574,275, filed Aug. 28, 1990now U.S. Pat. No. 5,165,906.

FIELD OF THE INVENTION

The invention relates to a synthetic crystalline zeolite of the mazzitetype, a process for its synthesis and its use in adsorption and as acatalyst for the conversion of hydrocarbons.

BACKGROUND OF THE INVENTION

The zeolite known under the name mazzite was discovered in 1972 (GALLIet al. Contr. Mineral. and Petrol 45 99 (1974)) in the basalts of MountSemiol, Loire, France. The structure of the mazzite (Rinaldi et al. ActaCryst. B31 1603 (1975)) is described by an assembly of six columns ofgmelinite cages, which corresponds to two hexagonal faces, 9 squarefaces and three octogonal faces joined together with a displacement of±1/4.

The three-dimensional lattice is characterized by a system of threechannels which are parallel to the crystallographic axis c:

a--quasi-cylindrical channels bordered by 12-tetrahedral rings whosefree diameter is between 0.75 and 0.8 nm. The walls of these channelsare lined with ladders consisting of alternating rings made up of 4tetrahedrons of gmelinite cages and of rings made up of 5 tetrahedrons,which are the result of joining the columns;

b--secondary channels, located between the columns of gmelinite cagesand consisting of rings made up of 8 tetrahedrons forming a highlydistorted chair conformation, and

c--channels inside the columns made up of cages, accessible via therings consisting of 6 tetrahedrons.

The zeolites of the mazzite type are characterized by a unique X-raydiffraction pattern.

The best-known and best-characterized examples of this type of zeoliteare the zeolites omega and ZSM-4. They are solids which are of interestfor adsorption and catalysis.

The synthesis of the zeolite omega is described in U.S. Pat. No.4,241,036 (UNION CARBIDE) and U.S. Pat. No. 4,091,007 (MOBIL OIL).French Patent 2,074,007 (MOBIL OIL) and British Patent 1,297,256 (MOBILOIL) describe the synthesis of ZSM-4. A solid which is isostructuralwith zeolite omega and called LZ 202 is described in Patent ApplicationPCT WO 87/00,158 (UNION CARBIDE).

The synthesis of the zeolites omega and ZSM-4 is likewise described inthe scientific literature by Aiello et al. (J. Chem Soc. A 1970 1470),Dwyer et al. (J. Catal 59 263 (1979)), Cole et al. (Adv. Chem. Ser. 121583 (1973)), Perrotta et al. (J. Catal. 55 240 (1978)), Araya et al.(Zeolites 4 263 (1984)), Fajula et al. (Zeolites 7 203 (1987) ) andNicolas et al. (Stud. Surf. Sci. Catal. 37 115 (1987)).

Zeolite omega and zeolite ZSM-4 are prepared by hydrothermalcrystallization of reactive alkaline gels of aluminosilicates. With theexception of the one described in Patent Application WO 87/00,158, allsyntheses are carried out in the presence of an organic reagent, such astetramethylammonium (TMA), pyrrolidine, choline, diazobicyclooctane ortriethylenediamine. TMA is the most common and most selective organicreagent. These organic compounds are used as such or in the form ofhydroxide or in salt form. The addition of inorganic bases is necessaryto obtain the required level of alkalinity. For the synthesis of zeoliteomega and zeolite ZSM-4, the inorganic base used is sodium hydroxide.The reaction medium can contain lithium but does not tolerate potassiumor tolerates only a small amount of it (Aiello et al. J. Chem. Soc. A1970 1470 and Cole et al. Adv. Chem. Ser. 121 583 (1973)). In thepresence of potassium, offretite is in general obtained.

The general formula of the synthetic zeolites of the mazzite type intheir form as synthesized can be written in terms of moles of oxides:##EQU1## where m is an n-valent cation, in general sodium, and A is anorganic reagent carrying m positive charges.

According to a particular feature of the aluminosilicate, such as thezeolites, the cations and the organic compounds incorporated in thecourse of the synthesis can be removed and replaced by other cations.These operations are even necessary for freeing the pores of the zeoliteand generate catalytic activity.

The organic reagents are more conventionally removed by oxidativecalcination. The cations M are exchangeable for other cations in aqueousmedium by well-known methods. The most useful cations for catalysis arethe proton or cations belonging to the class of rare earths.

However, these exchange treatments induce an embrittlement of thezeolitic skeleton, especially during the operations which require theapplication of elevated temperatures in the presence of water vapour.The phenomenon of embrittlement is well known; it is caused by reactionswhich hydrolyze the Al-O-T bonds, where T is a cation of the lattice, ingeneral silicon, the reactions being catalysed by the protons which arepresent in the lattice to neutralize the excess negative chargeassociated with the presence of aluminium. In the course of prolongedhydrothermal treatments, the zeolite structure gradually decomposes andfinally becomes amorphous.

SUMMARY OF THE INVENTION

The main object of this invention is a synthetic crystalline zeolite ofthe mazzite type which retains its crystallinity and its characteristiccatalytic properties and adsorption behaviour after a prolongedhydrothermal treatment.

The second objective is a crystalline zeolite of the mazzite type inwhich a portion of the inorganic cations which were incorporated duringthe synthesis is non-exchangeable.

Accordingly, the invention relates to a new synthetic crystallinezeolite of the mazzite type whose stoichiometric composition, expressedin moles of oxides, corresponds to ##EQU2## where M represents at leastone exchangeable n-valent alkali metal or alkaline earth metal cation,preferably sodium or potassium, or a proton, A is an organic reagentcarrying m positive charges, preferably the tetramethylammonium ion, xis the fraction of non-exchangeable potassium cations, which is between0.1 and 0.8, preferably between 0.2 and 0.5, y is in the range from 3 to20, preferably from 4 to 10, and z is equal to one or zero,

the X-ray diffraction pattern corresponds to that described in Table 1

crystallinity and adsorption capacity in its protonated form areretained after a prolonged hydrothermal treatment.

We have named this new zeolite of the mazzite type MZ 34.

DETAILED DESCRIPTION OF THE INVENTION

The main peaks of the X-ray diffraction pattern of MZ 34 are listed inTable 1.

                  TABLE 1                                                         ______________________________________                                        X-ray diffraction pattern of the zeolite MZ-34                                d (10.sup.-1 nm)                                                                          I/I.sub.o × 100                                                                         hkl                                               ______________________________________                                        15.47        9              100                                               9.03        60              110                                               7.83        15              200                                               6.82        15              101                                               5.93        31              210                                               5.47         3              201                                               5.24         6              300                                               4.679       34              211                                               4.357        5              310 + 301                                         3.934       11              400                                               3.789       100             311 + 002                                         3.691       27              102                                               3.610       35              320                                               3.501       67              112 + 401                                         3.42        13              202 + 410                                         3.264        6              321                                               3.149       85              500 + 411                                         3.076       36              302                                               3.030       32              330                                               2.975       22              420                                               2.910       78              501 + 222                                         2.826        6              510 + 331                                         2.619       18              511                                               ______________________________________                                    

The diffraction pattern was obtained by the conventional method, usingcopper Kα₁ radiation and an amplifying detector. The intensities of thepeaks are recorded as a function of their position and expressed as 2theta, where theta is the Bragg angle. On this basis, the relativeintensities I/Io, where I is the intensity of a given peak and Io is theintensity of the most important peak, and the interplanar spacingsd(10⁻¹ nm) are determined. These values can slightly vary as a functionof parameters such as the degree of hydration of the zeolite, thetemperature or the type of apparatus used, but, as a general rule, theX-ray diffraction pattern is characteristic of the crystalline latticeof the zeolite.

In the zeolites of the omega or ZSM-4 type, the inorganic and organiccations introduced in the course of the syntheses can be removed by acombination of oxidative thermal treatments and exchange reactions bymethods well known in the art. They are described, for example, in thepublications of Perrotta et al. (J. Catal. 55 240 (1978)), of Cole etal. (Adv. Chem. Ser. 121 583 (1973)) or Weeks et al. (J. Chem. Soc.Faraday Trans. I 72 575 (1976)).

In contrast, MZ-34 contains x potassium ions per aluminium, which cannotbe exchanged by these methods.

Thus, after M-34 has been converted to its protonated form, it stillretains x potassium ions. Its potassium content does not decrease as aresult of the hydrothermal treatments.

The protonated form of MZ-34 is obtained by thermal decomposition of theorganic cation, exchange with an inorganic ammonium salt, followed bythermal decomposition of the ammonium salt.

In general, ammonium nitrate is used as inorganic ammonium salt. Theexchange requires a treatment under reflux of about 18 hours.

The thermal decomposition of the organic cation and the ammonium salttakes place by calcination at temperatures of 500°-600° C. in air over aperiod of about 5 hours.

The stoichiometric composition of the protonated form of MZ-34,expressed in moles of oxides, is

    1±0.5 [(1-x)H+x K].sub.2 O; Al.sub.2 O.sub.3, ySiO.sub.2 ; 0-20H.sub.2 O

where x and y have the abovementioned meanings.

Table 2 shows that after a treatment at 750° C. for 17 hours in thepresence of water vapour, the potassium level of MZ-34 remainsunchanged, while the initially low potassium level of zeolite omegadrops after the hydrothermal treatment of the acid form.

The remaining cation level is determined by chemical elemental analysis.

After potassium ions have been introduced into a zeolite omega or azeolite ZSM-4, they can subsequently be exchanged with a cation of anysalt. Thus, a zeolite omega exchanged with 2% by weight of potassium andtreated under reflux with a molar solution of ammonium nitrate, retainsonly 0.1% by weight of potassium after this exchange.

The potassium content of a zeolite MZ-34 containing 2% of potassiumremains unchanged after such a treatment.

This difference can be explained by the position of potassium in thecrystal lattices of zeolite omega and zeolite MZ-34. This position canbe deduced from the adsorption capacity of the two zeolites.

                  TABLE 2                                                         ______________________________________                                        Properties of the acid forms of zeolite                                       omega and zeolite MZ-34                                                       OMEGA               MZ-34                                                     Start-      17 h, 750° C. 17 h, 750° C.                         ing ma-     1.013 × 10.sup.5 Pa                                                                 Starting 1.013 × 10.sup.5 Pa                    terial      H.sub.2 O   material H.sub.2 O                                    ______________________________________                                        Crystal-                                                                              100     0           100    75                                         linity                                                                        Cyclo-  0.08    <0.005      0.08   0.06                                       hexane                                                                        adsorbed                                                                      (ml × g.sup.-1)                                                         Cracking                                                                              750     <10         700    650                                        rate of iso-                                                                  octane (1)                                                                    mmol/h/g                                                                      % by    0.1     <0.03       2      2                                          weight K                                                                      ______________________________________                                         (1) 380° C., sv = 11 h.sup.-1, 1.013 × 10.sup.5 Pa          

The non-exchangeable potassium ions of MZ-34 do not diminish itsadsorption capacity in any way. MZ-34 in its protonated form containing2% by weight of potassium adsorbs 80% by weight of cyclohexane, 10 to13% by weight of water at 25° C. and 1333.2 Pa and 3 to 6% of1,3,5-trimethylbenzene at 25° C. and a pressure of 2666.4 Pa. Theseresults prove that the free, pore diameter is at least 0.75 nm andcorresponds to the calculated void volume of the large channel (Breckand Grose Adv. Chem. Ser. 121 319 (1973)).

In contrast, in a zeolite omega into which 2% by weight of potassium isintroduced by ionic exchange, under the same measurement conditions areduction in its adsorption capacity can be observed. Thus, theadsorption capacity for cyclohexane goes from 8% by weight to 2 to 4%.

By virtue of its porous structure, only the large channel of the zeoliteis accessible to hydrocarbons (Chauvin et al. J. Catal. 111 94 (1988)).The exchange process, involving potassium of the zeolite omega, reducesthe adsorption capacity, suggesting that the ions introduced are locatedin the large channel.

On the other hand, in Mz-34, the entire volume of the large channel isunoccupied. Without wishing to be limited by theoretical considerations,we believe that the non-exchangeable potassium ions of Mz-34 are locatedinside the gmelinite cages of the structure, from where they would notbe able to leave because of their size. It should be kept in mind thatthe ionic radius of anhydrous potassium is 0.133 nm, compared with 0.097nm for sodium. Furthermore, these gmelinite cages are inaccessible byconventional methods of cationic exchange.

We do not wish to be limited by this explanation, but we believe thatthe hydrothermal stability of MZ-34 is due to the presence of thenon-exchangeable potassium ions. Indeed, the formation of an amorphousstructure of zeolite omega or zeolite ZSM-4 as a result of thehydrothermal treatments is caused by reactions, hydrolyzing the Al-O-Sibonds, which are promoted by the presence of protons neutralizing thenegative charge. In Mz-34, the non-exchangeable potassium ions blockthis hydrolytic process and thus stabilize the three-dimensionallattice.

The introduction of potassium ions into a zeolite omega or zeolite ZSM-4by exchange does not produce a stabilizing effect. These potassium ionsare removed in the course of the hydrothermal treatment.

In Table 2, the hydrothermal stability of a zeolite omega and a zeoliteMZ-34 in their protonated form are compared.

The protonated forms are obtained from the form as synthesized bycalcination at 500° C. in air, exchange under reflux for 18 hours with amolar solution of ammonium nitrate, followed by a second calcination.

The protonated forms are subjected to a hydrothermal treatment, whichconsists in heating at 750° C. for 17 hours under a water vapourpressure of 1.013×10⁵ Pa.

Under these conditions, MZ-34 retains 75% of its crystallinity, whilezeolite omega becomes amorphous. The crystallinity is measured by X-raydiffraction, comparing the ratio between the sums of the areas of thediffraction peaks of the zeolites before and after the hydrothermaltreatment.

Another evidence for retention of crystallinity in MZ-34 is itsadsorption capacity for cyclohexane, which remains at a value equal toat least 75% of its initial value.

Zeolite omega and zeolite MZ-34 have a comparable activity in thecracking reaction of isooctane at 380° C. and at atmospheric pressure.However, the main advantage of MZ-34 is its activity after thehydrothermal treatment, which is about 80% of the original value, whilezeolite omega becomes completely inactive.

This invention likewise relates to the synthesis of MZ-34.

The synthetic procedure comprises the following steps:

a) preparation of an alkaline solution in general containing sodiumhydroxide, potassium hydroxide and an organic reagent

b) addition of the source of silicon to the clear solution with constantand vigorous stirring, and continued stirring in general for 5 to 100hours until a liquid and homogeneous mixture is obtained. This operationis generally carried out at ambient temperature, but the dissolution ofthe source of silicon can be accelerated by keeping the entire batch ata temperature between 30° and 50° C.

c) allowing the alkaline suspension of this source of silicon to stand

d) addition of the source of aluminium with vigorous stirring, thenrapidly bringing the mixture to the crystallization temperature, whichin general is between 80° and 150° C. This temperature is maintaineduntil the formation of a crystalline phase showing the X-ray diffractionpattern given in Table 1. The crystallization time generally variesbetween 20 and 150 hours.

The molar ratios in the crystallization gel are summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                        Molar ratios in the crystallization gel for the                               formation of zeolite MZ-34                                                                                Particularly                                              Broad     Preferred preferred                                                 range     range     range                                             ______________________________________                                        SiO.sub.2 /Al.sub.2 O.sub.3                                                              10-100     20-80     28-60                                         H.sub.2 O/Al.sub.2 O.sub.3                                                               100-1000    500-2000  600-1300                                     OH.sup.- /SiO.sub.2                                                                     0.2-1       0.5-0.9   0.65-0.85                                     Na/T.C.   0.4-0.9     0.4-0.9   0.4-0.9                                       K/T.C.    0.10-0.50   0.10-0.50 0.10-0.50                                     TMA/T.C.  0.02-0.2    0.02-0.2  0.05-0.15                                     ______________________________________                                         T.C.: Na + K + TMA                                                       

It is of interest to note that, although the literature teaches that thezeolites of the omega and ZSM-4 type cannot be obtained in the presenceof potassium ion proportions greater than about 10%, relative to thetotal amount of cations (Aiello et al. J. Chem. Soc. A 1970 1470 andCole et al. Adv. Chem. Ser. 121 583 (1973)), we have found a methodwhich makes it possible to synthesize a zeolite of the mazzite type inmedia where the potassium ions can represent up to 50% of the inorganicions.

On the other hand, and we do not wish to be limited by this explanation,we believe that the formation of MZ-34 is in part due to the fact thatthe aluminium is introduced into a mixture in which the source ofsilicon has been predigested, i.e. a medium containing structuredsilicate ions. Indeed, in the synthetic procedures described in theprior art, the source of aluminium is in general dissolved in thealkaline source before the silica. If that methodology is applied, evenin combination with the stoichiometric ratios described in Table 3,offretite is formed.

The sources of silicon and aluminium can be of any type. Of the sourcesof silicon, silicates, solid silicas, colloidal silicas and silica solsand gels can be mentioned. As source of aluminium, aluminium salts,metallic aluminium, aluminium oxides, clays and synthetic and naturalzeolites can be mentioned. It is preferred to use a source of aluminiumof low solubility, such as a synthetic or natural crystallinealuminosilicate. This result could confirm the hypothesis according towhich the formation of MZ-34 is linked to a slow incorporation ofaluminium in dissolved silicates.

Of the organic reagents, tetramethylammonium ions, introduced in theform of their hydroxide or one of their salts, are preferably used.

The duration of the synthesis can be shortened by applying methods wellknown in the art, such as seeding or ageing. Seeding is understood tomean the addition to the reaction mixture of 1 to 20% of zeolite MZ-34crystals, or 1 to 30% by volume of an MZ-34 gel which has not yet begunto crystallize, or 1 to 30% of mother liquors of a previous synthesis.If the seeding takes place by means of an MZ-34 gel which has not yetbegun to crystallize, it is possible to use a reaction mixture whichotherwise is devoid of tetramethylammonium ions, such as described inU.S. Pat. No. 3,947,482 of GRACE. Ageing consists in maintaining thereaction mixture at a temperature below the crystallization temperaturefor periods between 1 hour and 20 days.

The protonated form of MZ-34 is a very efficient catalyst for a varietyof processes of hydrocarbon conversion, such as cracking, isomerization,alkylation, reforming, polymerization or dewaxing. It is possible toreplace a portion of the protons by cations having a catalytic effect,such as, for example, cations belonging to the class of rare earths.

The catalytic activity of MZ-34 is comparable to that of zeolite omegaor zeolite ZSM-4, its main advantage being that it maintains thisactivity after even prolonged hydrothermal treatments.

The examples which follow illustrate the invention, without, however,limiting it.

EXAMPLE 1

This example describes a general method for synthesizing zeolite MZ-34according to the invention.

In a beaker containing 50 ml of deionized water, 2.62 g of sodiumhydroxide pellets (NaOH), 1.92 g of potassium hydroxide pellets (KOH,85.5% pure) and 0.97 g of tetramethylammonium hydroxide pentahydrate(TMAOH, 5H₂ O) are dissolved.

8.10 g of powdered silica (Cecagel, 80.6% of SiO₂, 19.4% of H₂ O) arethen added to this alkaline solution with vigorous stirring. After 10hours and after a liquid and homogeneous mixture has been obtained, thesource of aluminium is added in the form of 1.85 g of zeolite type Y(Na₂ O; Al₂ O₃ ; 4.6 SiO₂ ; 18.7 H₂ O), and the mixture is stirred forfive hours. The mixture then has the following overall composition:

    23(0.66 Na+0.29 K+0.05 TMA).sub.2 O; Al.sub.2 O.sub.3 ; 53 SiO.sub.2 ; 1295 H.sub.2 O

The mixture is transferred to a stainless steel reactor equipped withinternal stirring and is heated at 115° C. for 24 hours at a rotatingspeed of the stirrer of 140 revolutions/minute.

The MZ-34 crystals are recovered by filtration, washed and dried in anoven at 70° C. at ambient atmosphere. Their X-ray diffraction pattern isidentical to that reported in Table 1 and is characteristic of a zeoliteof the mazzite type of the following structural parameters:a=1.821+0.001 nm, c=0.7608+0.004 nm and a unit cell volume of 218.27nm³.

Its composition in terms of moles of oxides, such as determined byelemental analysis, is as follows:

    (0.3 Na+0.45 K+0.25 TMA).sub.2 O, Al.sub.2 O.sub.3 ; 9.55 SiO.sub.2 ; 6.5 H.sub.2 O

This solid is calcined at 550° C. for 5 hours in air to decompose theTMA ions, then exchanged three times in a 1M ammonium nitrate solutionunder reflux (NH₄ NO₃) to exchange the inorganic cations for ammoniumions. Upon completion of the treatment, the solid is analyzed. Itcontains 2% by weight of potassium, which indicates that 55% of theinitial potassium ions are non-exchangeable.

An acid catalyst was prepared from this exchanged solid by thermaldecomposition (500° C., 4 hours, 100 ml/mn of air) of the ammonium ionsto give protons. The composition of the catalyst in terms of moles ofoxides is as follows:

    (0.75 H+0.25 K).sub.2 O, Al.sub.2 O.sub.3, 9.55 SiO.sub.2, 7.2 H.sub.2 O

This solid adsorbs 12.6% by weight of water (1333.3 Pa 25° C.), 6.2% byweight of cyclohexane and 4.5% by weight of mesitylene (2666.6 Pa 25°C.). When applied as catalyst for the cracking of isooctane(2,2,4-trimethylpentane) at 380° C. at atmospheric pressure (1.013×10⁵Pa) and a space velocity (weight of hydrocarbon/weight of catalyst xtime in hours) of 11 h⁻¹, it results in a cracking rate of 750 mmol/hxg.The catalyst was also evaluated after a hydrothermal treatment. Thehydrothermal treatment consists in heating at 750° C. under a watervapour pressure of 1.013×10⁵ Pa for 17 hours. After this treatment, itretains 75% of its crystallinity according to X-ray diffraction, itsadsorption capacity for cyclohexane is 5% by weight and its activity forthe cracking of isooctane is 650 mmol/hxg under the conditions describedabove.

EXAMPLE 2

This example illustrates the preparation of zeolite MZ-34 in thepresence of a previously aged gel with a controlled supply of aluminium.

i) A gel having an overall stoichiometry of 4(0.8 Na+0.2 TMA)₂ O, Al₂O₃, 9.2 SiO₂, 156 H₂ O is prepared from 160 ml of deionized water, 10.47g of NAOH pellets, 17.1 g of TMAOH×5H₂ O, 35.2 g of powdered silica(non-porous silica, 91.2% of SiO₂, 8.8% of H₂ O) and 9.7 g of sodiumaluminate (NaAlO₂), using the procedure and order of addition of thereagents described in Example 1. After homogenization, the mixture isplaced in a polypropylene bottle, sealed and kept in an oven at 50° C.for 23 days without stirring.

ii) A second mixture having the composition: 4 Na₂ O; K₂ O; 15 SiO₂ ;377.3 H₂ O is prepared from 360 ml of water, 17.78 g of NAOH, 7.25 g of85% strength KOH and 60.1 g of powdered silica (Cecagel, 80.6% of SiO₂,19.4% of H₂ O).

After digestion of the silica, 90 ml of the aged gel described in i) isadded to this fresh gel. The entire mixture is transferred to a stirredautoclave of 1 liter capacity, which is raised to 115° C.

iii) After the temperature in the inside of the autoclave has reached115° C., an alkaline aluminate solution having a concentration of 0.46mol/l of sodium aluminate and 1.23 mol/l of potassium hydroxide preparedfrom 700 ml of water, 26.2 g of NaAlO₂ and 56.5 g of 85% strength KOH isadded to the system by means of a metering pump. The output of the pumpis set to such a value that a constant concentration of dissolvedaluminium in the autoclave of 3 mmol/l is maintained.

Since the crystal growth, which consumes aluminium, and the growthsurface increase with time, the output of the pump is regularly adjusted(between 0.6 ml/h at the beginning of the reaction and 19.2 ml/h at theend of the addition).

After 33 hours, the solid formed is recovered by filtration, washed anddried in a drying cabinet. Its X-ray diffraction pattern shows thecharacteristics of a zeolite of the mazzite type (Table 1) and itschemical composition is as follows:

    (0.45 Na+0.30 K+0.25 TMA).sub.2 O; Al.sub.2 O.sub.3 ; 10.4 SiO.sub.2 ; 13.4 H.sub.2 O)

After calcination to remove the TMA ions and three times of exchangeunder reflux with a 1M solution of NH₄ NO₃, the zeolite has a residualsodium content of 1.7% by weight, which indicates that 78% of thepotassium ions introduced during the course of the synthesis arenon-exchangeable.

Five grams of this exchanged solid were calcined in air at 500° C., inorder to obtain the protonated form, then treated in the presence ofwater vapour at 1 atmosphere at 750° C. for 17 hours. After thehydrothermal treatment, the solid has adsorbed 5.8% by weight ofcyclohexane and 4% by weight of mesitylene. Its crystallinity, asdetermined from the X-ray diffraction pattern, is 78%.

EXAMPLES 3 to 6

These are examples in which the procedure of Example 1 was followed toprepare zeolite MZ-34, but where the proportion of the reagents used hadbeen changed. (In the table in which these examples are summarized, T.C.denotes the total amount of cations and represents the sum of Na+K+TMA).

    ______________________________________                                        Example                                                                              TC      Na      K    TMA   SiO.sub.2                                                                           H.sub.2 O                                                                           OH.sup.-                        ______________________________________                                               Al.sub.2 O.sub.3                                                                      TC      TC   TC    Al.sub.2 O.sub.3                                                                    Al.sub.2 O.sub.3                                                                    SiO.sub.2                       3      21.9    0.528   0.42 0.052 51.7  1261  0.81                            4      11.6    0.562   0.39 0.05  29.7   625  0.77                            5      20.2    0.714   0.233                                                                              0.052 51.3  1205  0.75                            6      21.7    0.670   0.276                                                                              0.054 56.1  1247  0.70                            ______________________________________                                    

After reaction of 24 hours at 115° C., the solids obtained in Examples3-6 show the X-ray diffraction pattern of zeolite MZ-34 and contain 1.3to 2.2% by weight of potassium.

The subsequent treatments of calcination+exchange, such as describedabove, lead to non-exchangeable potassium contents of 1 to 1.8% byweight.

EXAMPLE 7 Comparison

This example shows that the use of the methods known in the art(addition of the source of aluminium to the alkaline solution) and theuse of a soluble source of aluminium (sodium aluminate) lead tooffretite and not to MZ-34.

An alkaline solution containing 50 ml of deionized water, 2.67 g ofNAOH, 1.96 g of 85% KOH and 1.022 g of TMAOH×5H₂ O is prepared, and0.525 g of sodium aluminate are dissolved therein. After completedissolution, 8.9 g of powdered silica (Cecagel, 80.6% of SiO₂, 19.4% ofH₂ O) are added with vigorous stirring. The mixture is transferred to asteel autoclave, stirred at a speed of 150 revolutions/minute and heatedat 115° C. for 24 hours.

The solid fraction is recovered by filtration, washed and dried, and itsX-ray diffraction pattern corresponds to that of offretite:

X-ray diffraction pattern of the zeolite obtained in Example 7(comparison)

    ______________________________________                                        d (10.sup.-1 nm)                                                                             I/Io × 100                                               ______________________________________                                        11.41          50                                                             7.54            8                                                             6.61           36                                                             6.3             5                                                             5.73           15                                                             4.564          25                                                             4.335          56                                                             3.931           3                                                             3.82           38                                                             3.763          100                                                            3.57           44                                                             3.314          29                                                             3.181          14                                                             3.157          21                                                             3.933           9                                                             2.868          79                                                             2.850          98                                                             2.683          23                                                             2.504          15                                                             2.488          17                                                             2.210          13                                                             2.119           9                                                             2.103           6                                                             2.086           6                                                             ______________________________________                                    

We claim:
 1. A synthetic zeolite of the mazzite structure whosestoichiometric composition, expressed as moles of oxides corresponds to:

    1±0.5(1-x) (M.sub.2n +zA.sub.2/m))O+XK.sub.2 O), Al.sub.2 O.sub.3, ySiO.sub.2, (0-20)H.sub.2 O

where M represents at least one exchangeable n-valent element selectedfrom the group consisting of alkali metals, alkaline earth metals, andprotons, A is an organic reagent carrying an m positive change, x is thefraction of non-exchangeable potassium cations, which is in the range of0.1 to 0.8, y is in the range from 3 to 20 and z is equal to one orzero, wherein the capacity of the zeolite to crystallize and adsorb inits protonated form is retained after hydrothermal treatment.
 2. Zeoliteaccording to claim 1 having the following X-ray diffraction pattern:

    ______________________________________                                        d (10.sup.-1 nm)                                                                          I/I.sub.o × 100                                                                         hkl                                               ______________________________________                                        15.47        9              100                                               9.03        60              110                                               7.83        15              200                                               6.82        15              101                                               5.93        31              210                                               5.47         3              201                                               5.24         6              300                                               4.679       34              211                                               4.357        5              310 + 301                                         3.934       11              400                                               3.789       100             311 + 002                                         3.691       27              102                                               3.610       35              320                                               3.501       67              112 + 401                                         3.42        13              202 + 410                                         3.264        6              321                                               3.149       85              500 + 411                                         3.076       36              302                                               3.030       32              330                                               2.975       22              420                                               2.910       78              501 + 222                                         2.826        6              510 + 331                                         2.619       18             
 511.                                              ______________________________________                                    


3. Zeolite according to claim 1, wherein x is in the range of 0.2 to0.5.
 4. Zeolite according to claim 1, wherein y is in the range from 4to
 10. 5. Zeolite according to claim 1, wherein A is atetramethylammonium ion.
 6. Zeolite according to claim 1, wherein Mrepresents a mixture of sodium and potassium.
 7. Zeolite according toclaim 1, wherein M represents a proton z is equal to zero.
 8. Zeoliteaccording to claim 7, wherein the zeolite retains at least 75% of itscrystallinity after heating at 750° C. for 17 hours under a water vaporpressure of 1.013×10⁵ Pa.
 9. In a process for the conversion ofhydrocarbons which comprises contacting a hydrocarbon feed with azeolite under conditions effective to convert said hydrocarbon feed to ahydrocarbon product different from said hydrocarbon feed, theimprovement which comprises using as said zeolite a mazzite zeolitewhose stoichiometric composition, expressed in moles of oxidescorresponds to:

    1±0.5(1-x) (M.sub.2/n +zA.sub.2/m)O+XK.sub.2 O), Al.sub.2 O.sub.3, ySiO.sub.2, (0-20)H.sub.2 O

where M represents at least one exchangeable n-valent element selectedfrom the group consisting of alkali metals, alkaline earth metals, andprotons, A is an organic reagent carrying an m positive charge, x is thefraction of non-exchangeable potassium cations, which is in the range of0.1 to 0.8, y is in the range from 3 to 20 and z is equal to one orzero, wherein the capacity of the zeolite to crystallize and adsorb inits protonated form is retained after hydrothermal treatment.